1. Field of the Invention
The present invention relates to a shot peening method.
2. Description of Related Art
One of the techniques for bending a metal plate is stamping. Stamping is a technique in which an upper die and a lower die sandwich a workpiece and bend the workpiece. Japanese Unexamined Patent Application Publication No. 2006-312176 discloses a technique for stamping a metal plate, in particular, a technique for preventing springback in a metal plate which has been bent.
Further, one of the metal processing techniques is shot peening. Shot peening is a type of cold working and is a technique for improving fatigue strength under repeated loading by causing a shot material which is iron or non-ferrous metal spheres to hit a metal surface at a high speed.
As described above, when a metal plate (a workpiece) is stamped, the workpiece is sandwiched by an upper die and a lower die and then bent. At this time, springback, which is a phenomenon in which a part which has been bent tending to return to its original shape, may occur. Moreover, the workpiece itself may twist after being stamped. The present inventor has found a problem that when the springback or twist occurs in the stamped workpiece, a form accuracy of the stamped workpiece is lowered.